AP Physics C: Mechanics Retarding & Drag Forces

We last learned about friction, the sort of ‘anti-force that prevents objects from achieving velocity. A more general topic of frictional forces that occurs when an object has achieved velocity is retarding forces and drag. Friction can be thought of as the minimal force required to get an object to move, but once it is moving, drag and other factors come into play that try and stop the object. These forces play an important role in things like gas mileage of a car and terminal velocity of falling objects. In some problems later on youll see there are two kinds of friction: static and kinetic. Static friction is the friction youve learned, or the ‘standstill friction, and drag is the kinetic friction, or the moving friction.

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In many of the downloadable lecture slides, several identical slides in a row appear (e.g. in Mech-10 (Retarding and Drag forces), slide Velocity-as-a-Function-of-Time_332 duplicates slide 747 and slide 916, and all have only a title. Also, in some cases, the slide is unreadable because it is distorted or partially off the page, e.g. slide 748 in this lecture. Is there any way to get better copies of the slides, that contain all the information from the lecture?

1 answer

Last reply by: Professor Dan FullertonThu Aug 4, 2016 1:26 PM

Post by Peter Keon August 1, 2016

For Example 2, at 28:18 I understand why you put -k next to the dv but how did you get -1/k on the outside of the integral?

1 answer

Last reply by: Yuhuan YeSun Jan 3, 2016 9:58 PM

Post by Yuhuan Yeon January 3, 2016

Hi Mr.Fullerton,I have a question on "velocity as a function of time", just right before you integrate the equation, where did the "d" in "b/m*dt" go?

1 answer

Last reply by: Professor Dan FullertonWed Oct 15, 2014 2:14 PM

Post by Scott Beckon October 15, 2014

Is the Ap C Physics exam different from Ap physics 1 and 2? Could this course also prepare for the algebra based ap physics exams excluding optics and modern physics?

3 answers

Last reply by: Scott BeckWed Oct 15, 2014 1:25 PM

Post by Scott Beckon October 15, 2014

Why isn't the equation for net force in y direction Vo - mg + kv=ma? Since the ball was launched upward, and since the equation would be written Vo - mg -Fdrag but since Fdrag is -kv, to negatives make a positive turning it into Vo-mg+kv=ma?? Why isn't Vo included in the equation from the free body diagram?

Retarding & Drag Forces

When the frictional force is a function of an object’s velocity, the frictional force is known as a drag, or retarding, force.

Typically we assume the drag force takes the form F=bv or F=cv^2, where b and c are constants.

Once an object reaches its maximum velocity, when the net force on the object is zero, we say the object has reached its terminal velocity.

To find velocity as a function of time, write your Newton’s 2nd Law equation in the form of a differential equation. Standard bodies falling through the air with air resistance can typically be solved using the method of separation of variables.

Retarding & Drag Forces

Lecture Slides are screen-captured images of important points in the lecture. Students can download and print out these lecture slide images to do practice problems as well as take notes while watching the lecture.

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